Indention error on line 23 but I don't understand why it's an error

import random

COLORS = ['yellow', 'red', 'black', 'green']


class Monster:
    min_hit_points = 1
    max_hit_points = 1
    min_experience = 1
    max_experience = 1
    weapon = 'sword'
    sound = 'roar'

    def __init__(self, **kwargs):
        self.hit_points = random.randint(self.min_hit_points, self.max_hit_points)
        self.experience = random.randint(self.min_experience, self.max_experience)
        self.color = random.choice(COLORS)

        for key, value in kwargs.items():
            setattr(self, key, value)


  def __str__(self):
    return '{} {}, HP: {}, XP: ()'.format(self.color.title(),
                                  self.__class__.__name__,
                                  self.hit_points,
                                  self.experience)


    def battlecry(self):
        return self.sound.upper()

class Goblin(Monster):
  max_hit_points = 3
  max_experience = 2
  sound = 'squeak'


class Troll(Monster):
  min_hit_points = 3
  max_hit_points = 5
  min_experience = 2
  max_experience = 6
  sound = 'growl'


class Dragon(Monster):
  min_hit_points = 5
  max_hit_points = 10
  min_experience = 6
  max_experience = 10 
  sound = 'raaaaaaaaaar'